Centrifugal type of turbomachinery, such as centrifugal compressors, centrifugal pumps or centrifugal fans, adopts a centrifugal machine to impel working fluid. In general, a centrifugal machine consists of two principal parts: an impeller(s), which forces the working fluid to flow into a rotary motion by impelling action, and a volute casing, which directs the working fluid to the impeller(s) and leads the working fluid away under a higher pressure. A general structure of a conventional centrifugal machine 1 is as shown in FIG. 1. The centrifugal machine 1 comprises a suction chamber 11, a centrifugal impeller 12, a diffuser 13, a volute casing 14 and an impeller shaft 15. In the operation thereof, a working fluid enters the centrifugal machine 1 via the suction chamber 11. The impeller shaft 15 is driven to rotate the centrifugal impeller 12 at high speed for enhancing the kinetic energy of the working fluid. Therefore the kinetic energy of the accelerated working fluid can be converted into pressure energy via the deceleration and diffusion function of the diffuser 13 and the volute casing 14, and the higher pressure working fluid can further be ejected from the outlet of the centrifugal machine 1.
However, for the operation of the centrifugal machine 1, the pressure variation of the working fluid flowing with high velocity and the blades rotated at high speed will resulted in considerable noises. Generally, the noises contain high level tonal noises which will affect people's hearing. The aforementioned centrifugal impeller 12 is shown in FIGS. 2A and 2B. A plurality of blades 121 is arranged on the main body of the impeller 122, wherein the plurality of blades 121 surrounds the outer circumference of the main body of the centrifugal impeller 122, and the blades are arranged equiangularly (A1) and axisymmetrically to the shaft bore 123 (for passing through the impeller shaft 15). Consequently, when the working fluid flows from the inlet to the outlet of the impeller 12 via the passage, the diffuser 13 and volute casing 14, the noises will be generated due to periodical pressure and velocity pulsation caused by the rotation effect of impeller 12 and the geometry effect of blades 122. As shown in the noise spectrum in FIG. 3, the noise spectrum is distributed on the dominant frequency, the blade passing frequency, (the rotation speed of the impeller multiplied by the number of the blades) and harmonic frequencies of the centrifugal impeller. Generally, there is a considerably concentrated noise energy on the blade passing frequency of the impeller. This is why the operation of conventional centrifugal impellers always has a very high noise level. The noises caused by the centrifugal machine mainly comprise broadband noise and discrete tones noise. The broadband noise is generated because of the pressure pulsation caused by the peeling off of the boundary layer of the turbulent flow. The discrete tones noise is generated because of the periodical vibration of the equiangular blades, which relates to the blade passing frequency (the number of the blades multiplied by the rotation speed) of the impeller.
Consequently, the noise problem in this kind of machine is solved by respectively reducing the broadband noise and discrete tones noise. However, it is difficult for the practical design to reduce the broadband noise by changing the hydrodynamics or aerodynamics design of the elements for achieving better design of flow field and machine efficiency. Because the centrifugal machine needs to be driven and adjusted in a wide range, it is not easy to acquire the parameters of operation in a wide range and high efficiency. That will be the key issue for the hydrodynamics or aerodynamics design.
Other conventional methods for reducing discrete tones noise are also adopted, such as U.S. Pat. No. 3,635,579 as shown in FIG. 4. A soundproof casing 20 is additionally arranged outside the volute casing for reducing the operation noise of the impeller of the centrifugal machines. However, this method can't meet the requirements of batch producing because of its disadvantages of complicated structure and high cost.
An alternative method is disclosed in U.S. Pat. No. 4,411,592 as shown in FIG. 5, wherein an absorber material 25 is additionally arranged in a runner of an erect wall of the diffuser and the outlet of the impeller, for reducing noise. Similar designs are also disclosed in U.S. Pat. Nos. 4,504,188 and 5,249,919. Although noise can be reduced in such designs, the impedance of the runner will thus increase, and the operating efficiency will thus degrade, which can not meet practical requirements either.
It is the key issue to design a type of centrifugal impeller for the conventional centrifugal machines not only to solve the operating noise problems but also to meet the requirement of design cost and flow impedance.